Carrier substrate, method of manufacturing the same, and method of manufacturing flexible display device using the carrier substrate

ABSTRACT

A carrier substrate includes: a base substrate; a first coating layer on a first surface of the base substrate; and a second coating layer on a second surface of the base substrate. The thermal expansion coefficients of the first coating layer and the second coating layer are greater than a thermal expansion coefficient of the base substrate, and a thickness of the first coating layer is different from a thickness of the second coating layer.

CROSS-REFERENCE TO RELATED PATENT APPLICATION

This application claims priority to and the benefit of Korean PatentApplication No. 10-2012-0096788, filed on Aug. 31, 2012, in the KoreanIntellectual Property Office, the disclosure of which is incorporatedherein in its entirety by reference.

BACKGROUND

1. Field

Embodiments of the present invention relate to a carrier substrate, amethod of manufacturing the carrier substrate, and a method ofmanufacturing a flexible display device using the carrier substrate.

2. Description of Related Art

Recently, display devices have been replaced with thin flat paneldisplay devices that are portable and may be implemented to have largescreen characteristics. Organic or inorganic light-emitting displaydevices used as flat panel display devices may be self-emissive displaydevices having wide viewing angles, high contrast ratios, and highresponse speeds, and thus, are regarded as next-generation displaydevices. Organic light-emitting display devices, including an emissionlayer formed of an organic material, may have improved (or excellent)luminosity, driving voltage, and response speed characteristics relativeto inorganic light-emitting display devices, and may realize a colorimage. Organic light-emitting display devices may also be implemented tobe flexible using a plastic substrate with good flexibility.

When the plastic substrate is highly flexible, the plastic substrateshould be supported during a process of manufacturing a flat paneldisplay device. Accordingly, the plastic substrate may be bound to acarrier substrate made of glass while the flat panel display device ismanufactured. However, due to different thermal expansion coefficientsbetween the glass carrier substrate and the plastic substrate, warpagemay occur while the display device is formed on the plastic substratethrough a high-temperature process. This may cause problems, such aspattern errors, fracture of the carrier substrate, and delamination ofthin films.

SUMMARY

Aspects of embodiments of the present invention provide a carriersubstrate able to suppress warpage in a process of manufacturing aflexible display device, a method of manufacturing the carriersubstrate, and a method of manufacturing a flexible display device usingthe carrier substrate.

According to an aspect of the present invention, there is provided acarrier substrate including: a base substrate; a first coating layer ona first surface of the base substrate; and a second coating layer on asecond surface of the base substrate. The thermal expansion coefficientsof the first coating layer and the second coating layer are greater thana thermal expansion coefficient of the base substrate, and a thicknessof the first coating layer is different from a thickness of the secondcoating layer.

The thickness of the first coating layer may be from about 6 μm to about8 μm, and the thickness of the second coating layer may be from about 8μm to about 12 μm.

The first coating layer and the second coating layer may includepolyimide.

The base substrate may include glass.

According to an aspect of the present invention, there is provided amethod of manufacturing a carrier substrate, the method including:forming a first coating layer on a first surface of a base substrate soas to cause a warpage of the base substrate; and forming a secondcoating layer on a second surface of the base substrate so as tocompensate for the warpage of the base substrate. The first coatinglayer and the second coating layer include polyimide, and a thickness ofthe first coating layer is different from a thickness of the secondcoating layer

The thickness of the second coating layer may be greater than thethickness of the first coating layer.

The thickness of the first coating layer may be from about 6 μm to about8 μm, and the thickness of the second coating layer may be from about 8μm to about 12 μm.

The first coating layer and the second coating layer may be formed usingspin coating.

The base substrate may include glass.

According to an aspect of the present invention, there is provided amethod of manufacturing a flexible display device, the method including:preparing a carrier substrate; forming a display unit on the carriersubstrate; and encapsulating the display unit. The preparing of thecarrier substrate includes: forming a first coating layer on a firstsurface of a base substrate so as to cause a warpage of the basesubstrate; and forming a second coating layer on a second surface of thebase substrate so as to compensate for the warpage of the basesubstrate. A thickness of the second coating layer is greater than athickness of the first coating layer, and the display unit is formed onthe second coating layer.

The first coating layer and the second coating layer may includepolyimide.

The thickness of the first coating layer may be from about 6 μm to about8 μm, and the thickness of the second coating layer may be from about 8μm to about 12 μm.

The base substrate may include glass, and thermal expansion coefficientsof the first coating layer and the second coating layer may be greaterthan a thermal expansion coefficient of the base substrate.

The display unit may include an active layer, and the active layer maybe formed by crystallizing amorphous silicon.

The method may further include separating the second coating layerhaving the display unit on an upper surface thereof from the basesubstrate.

The separating the second coating layer from the base substrate mayinclude irradiating a laser beam having a wavelength of from about 250nm to about 350 nm and an energy of from about 250 mJ/cm2 to about 350mJ/cm2 onto an interface between the second coating layer and the basesubstrate.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other features and advantages of the present inventionwill become more apparent by describing in detail exemplary embodimentsthereof with reference to the attached drawings in which:

FIG. 1 is a flowchart of a method of manufacturing a flexible displaydevice, according to an embodiment of the present invention; and

FIGS. 2 to 7 are schematic cross-sectional views illustrating the methodof manufacturing a flexible display device of FIG. 1.

DETAILED DESCRIPTION

As the invention allows for various changes and numerous embodiments,embodiments will be illustrated in the drawings and described in detailin the written description. However, this is not intended to limit thepresent invention to particular modes of practice, and it is to beappreciated that all changes, equivalents, and substitutes that do notdepart from the spirit and technical scope of the present invention areencompassed in the present invention. In the description of embodimentsof the present invention, certain detailed explanations of related artare omitted when it is deemed that they may unnecessarily obscure theessence of the invention.

Terms such as “first” and “second” are used herein merely to describe avariety of constituent elements, but the constituent elements are notlimited by the terms. The terms are used only for the purpose ofdistinguishing one constituent element from another constituent element.

It will be understood that when an element, layer or region is referredto as being “on” another element, layer or region, the element, layer orregion can be directly on another element, layer or region or there maybe intervening elements, layers or regions therebetween.

Expressions such as “at least one of,” when preceding a list ofelements, modify the entire list of elements and do not modify theindividual elements of the list.

Embodiments of the present invention will now be described more fullywith reference to the accompanying drawings, in which exemplaryembodiments of the invention are shown. In the drawings, the sameelements are denoted by the same reference numerals, and a repeatedexplanation thereof will not be given. The thicknesses of layers andregions may be enlarged for clarity. In addition, the thicknesses ofsome layers and regions may be exaggerated for convenience ofexplanation.

FIG. 1 is a flowchart of a method of manufacturing a flexible displaydevice, according to an embodiment of the present invention. FIGS. 2 to7 are cross-sectional views illustrating the method of manufacturing aflexible display device illustrated in FIG. 1. In particular, FIGS. 2and 3 illustrate a method of manufacturing a carrier substrate,according to an embodiment of the present invention. FIGS. 4 to 7illustrate a method of manufacturing a flexible display device using acarrier substrate manufactured according to an embodiment of the presentinvention.

Hereinafter, a method of manufacturing a carrier substrate, according toan embodiment of the present invention will be described with referenceto FIGS. 1 to 3, and a method of manufacturing a flexible displaydevice, according to an embodiment of the present invention, will bedescribed with reference to FIGS. 4 to 7.

Referring to FIG. 1, a method of manufacturing a carrier substrate 100(see FIG. 3), according to an embodiment of the present invention,includes forming a first coating layer 120 on a first surface of a basesubstrate 110 (Operation S11), and forming a second coating layer 130 ona second surface of the base substrate 110 (Operation S12).

The base substrate 110 may be formed of a transparent material so as totransmit a laser beam during a detachment process. The base substrate110 may also be formed of a rigid material so as to support a displaydevice that is formed on a surface thereof. For example, the basesubstrate 110 may be formed of glass including SiO₂ as a primaryingredient. Alternatively, the base substrate 110 may include at leastone of borosilicate glass, fused silica glass, or quartz glass.

The first coating layer 120 may be formed of a plastic material, forexample, polyimide with high thermal resistance that is durable againsthigh temperatures at which low-temperature polysilicon (LTPS) isprepared (as described below). The first coating layer 120 may be formedon the first surface of the base substrate, for example, through spincoating and then heating.

A thermal expansion coefficient of the first coating layer 120 may bedifferent from that of the base substrate 110. For example, the firstcoating layer 120 formed of a plastic material may have a larger thermalexpansion coefficient as compared with that of the base substrate 110formed of a glass material. Accordingly, once the first coating layer120 is formed on the first surface of the substrate 110, due to a highcompression stress of the first coating layer 120, the base substrate110 and the first coating layer 120 may be bent in the direction awayfrom the first surface of the base substrate 110 with the first coatinglayer 120 thereon, as shown in FIG. 2.

In this state, a second coating layer 130 may be formed on a secondsurface of the base substrate 110, as illustrated in FIG. 3.

Since used as a substrate for a flexible display device, the secondcoating layer 130 may be formed of polyimide having a high transmissionratio and high thermal durability. That is, the second coating layer 130may be formed of the same material as used for the first coating layer120. The second coating layer 130 may be formed on the second surface ofthe base substrate 110, for example, through spin coating and thenthermal treatment, as in the forming of the first coating layer 120.

The second coating layer 130 may be formed to be thicker than the firstcoating layer 120. When formed of the same material, the second coatinglayer 130 and the first coating layer 120 may have the same or similarthermal expansion coefficient. However, when the second coating layer120 has a larger thickness than the first coating layer 120, acompression stress of the second coating layer 130 may be larger thanthat of the first coating layer 120, thus compensating for a warpagecause by the first coating layer 120. That is, the warpage of thecarrier substrate 100 caused due to the formation of the first coatinglayer 120 may be compensated for or relieved by the formation of thesecond coating layer 130.

The second coating layer 130 may be formed on a second surface of thebase substrate 110 with the first coating layer 120 placed to contact asurface of a stage. A display unit, which will be described below, maybe formed on the second coating layer 130. That is, a thin filmtransistor (TFT) array (see FIG. 5) and an organic light-emitting diode(OLED) may be formed on the second coating layer 130.

To prevent warpage during a deposition process and improve (or ensure)stability of the deposition process, the second coating layer 130 may beformed to have a thickness (T2) of about 8 μm to about 12 μm. When thethickness (T2) of the second coating layer 130 is less than 8 μl orlarger than 12 μm, larger stress may be exerted on the second coatinglayer 130, and thus a thin film overlaying the second coating layer 130may be more likely delaminated. When the thickness (T2) of the secondcoating layer 130 is less than 8 μm, the second coating layer 120 maynot serve normally as a substrate of a flexible display device. When thethickness (T2) of the second coating layer 130 is larger than 12 μm, theflexible display device may be less flexible.

According to an aspect of the present invention, the compression stressof the second coating layer 130 and the compression stress of the firstcoating layer 120 are exerted in opposite directions, thus preventingwarpage of the carrier substrate 100. When the thickness of the secondcoating layer 130 is the above-mentioned range of about 8 μm to about 12μm, the first coating layer 120 may have a thickness (T1) of about 6 μmto about 8 μm for a balance of the opposite forces.

Warpage of the carrier substrate 100 during the manufacture of thecarrier substrate 100 and further during manufacture of a flexibledisplay device may be reduced so that the display unit 200 may be formedon the second coating layer 130 to be stable.

Although in the embodiment of FIGS. 2 and 3, the first coating layer 120and the second coating layer 130 are formed to have the same or similarthermal expansion coefficient, and have different thicknesses, thepresent invention is not limited to this embodiment. For example, thefirst coating layer 120 and the second coating layer 130 may have thesame or similar thicknesses. In this regard, to compensate for thecompression stress of the first coating layer 120, which is formed priorto the formation of the second coating layer 130, the second coatinglayer 130 may have a larger thermal expansion coefficient than that ofthe first coating layer 120.

Hereinafter, a method of manufacturing a flexible display device usingthe carrier substrate 100, according to an embodiment of the presentinvention will be described.

Referring to FIG. 4, in one embodiment a display unit 200 is formed onthe second coating layer 130 of the carrier substrate 100 (OperationS13).

FIG. 5 is a cross-sectional view of a display unit 200 according to anembodiment of the present invention. Referring to FIG. 5, a TFT arrayand an OELD are formed on the second coating layer 130.

Prior to the formation of the TFT array, a buffer layer 21 may be formedon an upper surface of the second coating layer 130 for planarizationand to block infiltration of impurities. The buffer layer 21 may beformed of SiO₂ and/or SiN_(x) by any of a variety of deposition methods,for example, plasma enhanced chemical vapor deposition (PECVD),atmospheric pressure CVD (APCVD), or low-pressure CVD (LPCVD).

A TFT may be formed on the buffer layer 21. The TFT is electricallyconnected to an OLED and operates the OLED. Although, as an embodiment,FIG. 4 illustrates a top gate type TFT with an active layer, a gateelectrode, and a source/drain electrode sequentially disposed upon oneanother, the present invention is not limited thereto. Any of a varietyof TFT types may be used.

A semiconductor layer (not shown) may be formed on the entire surface ofthe buffer layer 21. The semiconductor layer may be formed of aninorganic semiconductor material, such as amorphous silicon, or anorganic semiconductor material.

An amorphous silicon layer may be crystallized into polycrystallinesilicon by any of a variety of methods. Examples of the crystallizationmethods are solid phase crystallization (SPC), excimer lasercrystallization (ELC), metal induced crystallization (Nip, metal inducedlateral crystallization (MILC), and sequential lateral solidification(SLS).

Subsequently, the resulting polycrystalline silicon layer may bepatterned, and source and drain regions in edge regions of the patternedsilicon layer may be doped with impurities and then activated to form anactive layer 22 including a source region 22 s, a drain region 22 d, anda channel region 22 c therebetween.

A process for forming the active layer 22 from the semiconductor layermay include a high-temperature process performed at about 300° C. toabout 500° C. For example, with regard to Excimer Laser Annealing (ELA),the amount of hydrogen of amorphous silicon should be less than about10%. This is because when the amount of hydrogen in the amorphoussilicon layer is high, hydrogen may be generated upon irradiation of alaser beam for crystallization, the properties of polycrystallinesilicon may deteriorate, and thus, a TFT having desirable (or excellent)properties may not be manufactured. Thus, a high-temperature process atabout 300° C. to about 500° C. may be performed in order to reduce theamount of hydrogen in the amorphous silicon layer. In addition, in orderto form the active layer 22 by doping polycrystalline silicon withimpurities and activating the active layer 22, a high activationtemperature of about 400° C. may be used.

A gate insulating layer 23 may be formed of SiO₂, SiN_(x), or the likeon the active layer 22, and a gate electrode 24 may be formed in aregion (e.g., a predetermined region) of the gate insulating layer 23.The gate electrode 24 may be connected to a gate line (not shown) forapplying an on/off signal of a TFT.

An interlayer insulating layer 25 may be formed on the gate electrode24. A source electrode 265 and a drain electrode 266 (also shown as 26in FIG. 5) may be formed to respectively contact the source region 22 sand the drain region 22 d of the active layer 22 via a contact hole. Theresulting TFT may be covered and protected by a passivation layer 27.

The passivation layer 27 may be formed as an inorganic insulating layerand/or an organic insulating layer. The inorganic insulating layer mayinclude SiO₂, SiNx, SiON, Al₂O₃, TiO₂, Ta₂O₅, HfO₂, ZrO₂, BST, or PZT,and the organic insulating layer may include polymer derivatives havingcommercial polymers (PMMA and PS) and a phenol group, an acryl-basedpolymer, an imide-based polymer, an allyl ether-based polymer, anamide-based polymer, a fluorine-based polymer, a p-xylene-based polymer,a vinyl alcohol-based polymer, or a combination thereof. The passivationlayer 27 may be formed as a multi-stack including the inorganicinsulating layer and the organic insulating layer.

An OLED may be formed on the passivation layer 27.

According to one embodiment, the OLED includes a pixel electrode 31disposed on the passivation layer 27, an opposite electrode 33 facingthe pixel electrode 31, and an intermediate layer 32 interposedtherebetween. The display device 200 may be classified as a bottomemission type display device, a top emission type display device, a dualemission type display device, or the like. A bottom emission typedisplay device may include a light transmitting electrode as the pixelelectrode 31 and a reflective electrode as the opposite electrode 33. Atop emission type display may include a reflective electrode as thepixel electrode 31 and a semi-transmitting electrode as the oppositeelectrode 33. Although one embodiment is described with reference to thedisplay device 200 as a bottom emission type display device, the presentinvention is not limited thereto.

The pixel electrode 31 may be formed as a transparent layer using ITO,IZO, ZnO, or In₂O₃ having a high work function. The pixel electrode 31may be patterned to have an island form that corresponds to each pixel.Although not illustrated, the pixel electrode 31 may be connected to anexternal terminal (not shown) to serve as an anode.

A pixel-defining layer (PDL) 29 may be formed as an insulating layer onthe pixel electrode 31 so as to cover the pixel electrode 31. Afterforming an opening (e.g., a predetermined opening) in the PDL 29, theintermediate layer 32 may be formed in a region defined by the opening,which will be describe below.

The opposite electrode 33 may be formed of, for example, Li, Ca, LiF/Ca,LiF/AI, Al, Mg, or Ag that have a low work function. The oppositeelectrode 33 may be formed over the entire emission region in which animage is realized. The opposite electrode 33 may be connected to anexternal terminal (not shown) to serve as a cathode.

Polarities of the pixel electrode 31 and the opposite electrode 33 maybe switched.

In one embodiment the intermediate layer 32 includes an organiclight-emitting layer, which may be formed of a low-molecular weightorganic material or a polymer organic material. When the organiclight-emitting layer is formed of a low-molecular weight organicmaterial, a hole transport layer (HTL) and a hole injection layer (HIL)may be sequentially stacked below the organic light-emitting layertoward the pixel electrode 31, and an electron transport layer (ETL) andan electron injection layer (EIL) may be sequentially stacked on theorganic light-emitting layer toward the opposite electrode 33. Inaddition to these layers, various layers may be stacked on or below theorganic light-emitting layer if necessary.

When the organic light-emitting layer is a polymer organic layer formedof a large-molecular weight organic material, only a polymer HTL may bestacked on the organic light-emitting layer toward the pixel electrode31. The polymer HTL may be formed on an upper surface of the pixelelectrode 31 using, for example, poly(3,4-ethylenedioxythiophene)(PEDOT) or polyaniline (PANI) by inkjet printing, spin coating, or thelike.

Next, referring to FIG. 6, in one embodiment the display unit 200 isencapsulated so as to be protected from external moisture or air(Operation S14). In particular, the display unit 200 may be sealed byforming a thin encapsulation film 35 on the display unit 200.

The encapsulation film 35 may have, but is not limited to, a structurein which an inorganic material layer including, for example, siliconoxide or silicon nitride, and an organic material layer including, forexample, epoxy or polyimide, are alternately stacked. However, thepresent invention is not limited to this structure. In some embodiments,the display unit 200 may be encapsulated by using an encapsulationsubstrate. Examples of the encapsulation substrate are a glasssubstrate, a plastic substrate, or a stainless steel (SUS) substrate.

Next, referring to FIG. 7, in one embodiment a laser beam is irradiatedto separate the second coating layer 130 from the base substrate 110 ofthe carrier substrate 100 (Operation S15). For example, when irradiatedonto an interface between the second coating layer 130 formed ofpolyimide and the carrier substrate 100, a laser beam of a particularwavelength band and energy band may be absorbed by the polyimide polymerso that binding of polymer chains in the polyimide may be broken, andthus the second coating layer 130 may be delaminated from the carriersubstrate 100.

The wavelength band of the irradiated laser beam may be from about 250nm to 350 nm and the energy band may be from about 250 mJ/cm² to 350mJ/cm². When the wavelength band is less than 250 nm or greater than 350nm, bonds between polymer chains of the second coating layer 130 may notbe debonded to delaminate the second coating layer 130. When the energyband is less than 250 mJ/cm², bonds between polymer chains of the secondcoating layer 130 may not be debonded to delaminate the second coatinglayer 130. When the energy band is greater than 350 mJ/cm², othermembers may be damaged.

The second coating layer 130 separated from the base substrate 110 mayserve as a substrate of the flexible display device 10. The surface ofthe second coating layer 130 subjected to the laser irradiation may beprocessed by, for example, etching and washing, to remove the sootcaused by the laser irradiation.

As described above, according to one or more embodiments of the presetinvention, warpage of a carrier substrate during manufacture of a flatpanel display device may be suppressed, and thus flexible displaydevices may be manufactured using the carrier substrate with a higheryield.

While the present invention has been particularly shown and describedwith reference to exemplary embodiments thereof, it will be understoodby those of ordinary skill in the art that various changes in form anddetails may be made therein without departing from the spirit and scopeof the present invention as defined by the following claims and theirequivalents.

What is claimed is:
 1. A carrier substrate comprising: a base substrate;a first coating layer on a first surface of the base substrate; and asecond coating layer on a second surface of the base substrate, whereinthermal expansion coefficients of the first coating layer and the secondcoating layer are greater than a thermal expansion coefficient of thebase substrate, and a thickness of the first coating layer is differentfrom a thickness of the second coating layer.
 2. The carrier substrateof claim 1, wherein the thickness of the first coating layer is fromabout 6 μm to about 8 μm, and the thickness of the second coating layeris from about 8 μm to about 12 μm.
 3. The carrier substrate of claim 1,wherein the first coating layer and the second coating layer comprisepolyimide.
 4. The carrier substrate of claim 1, wherein the basesubstrate comprises glass.
 5. A method of manufacturing a carriersubstrate, the method comprising: forming a first coating layer on afirst surface of a base substrate so as to cause a warpage of the basesubstrate; and forming a second coating layer on a second surface of thebase substrate so as to compensate for the warpage of the basesubstrate, wherein the first coating layer and the second coating layercomprise polyimide, and a thickness of the first coating layer isdifferent from a thickness of the second coating layer.
 6. The method ofclaim 5, wherein the thickness of the second coating layer is greaterthan the thickness of the first coating layer.
 7. The method of claim 6,wherein the thickness of the first coating layer is from about 6 μm toabout 8 μm, and the thickness of the second coating layer is from about8 μm to about 12 μm.
 8. The method of claim 5, wherein the first coatinglayer and the second coating layer are formed using spin coating.
 9. Themethod of claim 5, wherein the base substrate comprises glass.
 10. Amethod of manufacturing a flexible display device, the methodcomprising: preparing a carrier substrate; forming a display unit on thecarrier substrate; and encapsulating the display unit, wherein thepreparing of the carrier substrate comprises: forming a first coatinglayer on a first surface of a base substrate so as to cause a warpage ofthe base substrate; and forming a second coating layer on a secondsurface of the base substrate so as to compensate for the warpage of thebase substrate, wherein a thickness of the second coating layer isgreater than a thickness of the first coating layer, and the displayunit is formed on the second coating layer.
 11. The method of claim 10,wherein the first coating layer and the second coating layer comprisepolyimide.
 12. The method of claim 10, wherein the thickness of thefirst coating layer is from about 6 μm to about 8 μm, and the thicknessof the second coating layer is from about 8 μm to about 12 μm.
 13. Themethod of claim 10, wherein the base substrate comprises glass, andthermal expansion coefficients of the first coating layer and the secondcoating layer are greater than a thermal expansion coefficient of thebase substrate.
 14. The method of claim 10, wherein the display unitcomprises an active layer, and the active layer is formed bycrystallizing amorphous silicon.
 15. The method of claim 10, furthercomprising separating the second coating layer having the display uniton an upper surface thereof from the base substrate.
 16. The method ofclaim 15, wherein the separating the second coating layer from the basesubstrate comprises irradiating a laser beam having a wavelength of fromabout 250 nm to about 350 nm and an energy of from about 250 mJ/cm2 toabout 350 mJ/cm2 onto an interface between the second coating layer andthe base substrate.